I'm not quite sure that this question has been asked, but I might also be misreading some earlier questions. Regardless, I am installing fiberglass insulation batts to go in my (sadly) 2×4 ceiling joist bays. The underside (ceiling side) will be faced, and once all the batts are in, I plan to cover some portions with OSB to make attic access/storage easier.
My attic has two vents at the peaks of the gables, and two Whirlybird vents, so it's definitely an unconditioned space. I have no reason to make it a conditioned space, but should I also install insulation in the rafter bays (like rigid foam cut to fit the bays) or under the rafters (like panels of rigid foam sitting on the rafters, leaving an air channel behind them)?
My guess is that this isn't useful, but I'm simply unsure.
EDIT: I should add that there are no soffit vents. The only attic vents are the gable peak vents and the Whirlybirds, which makes for a very hot attic in the summer.
EDIT: I'm in Redwood City, CA. Winter temperatures get close to freezing at night, though rarely to freezing or below, and easily up to and over 100 °F during the day in the summer.
Best Answer
You should not. Because the attic is vented, the upper insulation does almost nothing. In the summer at the attic will get much hotter than it does now, increasing cooling costs.
Also, you would be creating what's known as a hot roof. This can be damaging to the roof sheathing and shingles.
Most truss bottom cords are 2x4. This is normal, and the insulation usually reaches much higher than the tops of the ceiling joists. The usual routine is to raise the floor with additional framing to allow for more insulation.